Kyösti Kallio 0,60 Mk Finland 1973 – Flaw Or What?

The ink is a little smeared. Could it be caused when the sheet touched the backside of another sheet?

I think Don made a good point not to make it too complicated.

The lines on the back are quite sharp and appear to coincide with that on the front. There is very little evidence it is an offset. If printed on the web, the offset becomes even more unlikely.
If you are convinced it is a variety you should decide whether you are satisfied with your conviction and leave it, or whether you are willing to pay a fee and have it expertised. Before doing so, you might want to consult a specialist who can examine the stamp and tell you whether you are on the right track.

At best, we can tell you why it cannot be a variety. I think, in this case, there has not been a definitive argument it is a colour-changeling. From that point onwards - again as Don suggested - it requires an expert who can examine the stamp itself rather than pictures of it.

Thanks. I got the answer from the dealer who is part of Nordfrim. They told that they have never seen such a stamp before. Nothing else. Not interested..?

I contacted the Philatelist Assosiation. Let's see what they have to say.

If it really was a rare oddity, it should interest those who are in the business. Such oddities don't pop up every day in the Finnish Philately.

I don't expect it to be a rarity but am just interested to know what it is. The probability that it is a rarity is very low... but still possible...

How closely did you examine this, can you see the gold ink under the green, I use a 20x microscope when I need to see the finer details. If it was ink contamination, I would think you might be able to see the other color possibly the blue.

The other possibility is this just happened from improper storage, some type of paper or plastic against the front that had a chemical reaction over time.
I have seen this on some US stamps on more than one occasion.

I got the answer from a certifier:

"The gold colour turns green when the stamp has been in contact with plastic."

It seems a good explanation but...

There are two kind of stockbooks with plastic or pergamin strips. I have both stockbook types, but stamp was stored in a book with pergamin strips, so the change had taken place before I got the stamp about 15–20 years ago.



But...

Usually strips don't cover the whole stamp (depending on the stamp size), so if the stamp was kept in a stockbook with plastic strips, only the lower half would have been in contact with plastic. The are, however, also loose pages which cover almost the whole stamp.



Even if the stamp was kept in a loose page like above, what would explain the offset on the backside? Would the contact with plastic start a chemical prosess which makes the changed ink penetrate through the paper?

Leuchtturm claim that their mounts, strips, stockbooks etc. won't harm stamps.

@mike2006 No other colours, only green.

Chances are as you said that this probably happened before you got them.

From what I have seen on some U.S. stamps, is the bleed through of some inks is more common than the inks actually changing color.